Analysis of Effects of CO2 Injection on Coalbed Permeability: Implications for Coal Seam CO2 Sequestration

A proper understanding of permeability reductions of CH4-containing coal seams after CO2 injection is essential, as coal permeability is the key parameter influencing the efficiency of enhanced coalbed methane recovery with CO2 sequestration and theoretical research on it is lacking. The main objective of this study was to accurately quantify the effects of CO2 injection on coalbed permeability. Therefore, permeability decrease coefficients and permeability rebound and recovery pressures of a binary gas (CH4 + CO2) are proposed based on the Shi-Durucan and extended Langmuir models. Then, the trends of these parameters under the influence of the main influencing factors are detailed. Specifically, the permeability decrease coefficient increased with an increase in CO2 proportion and increased rapidly when the reservoir gas pressure was low. Permeability recovery pressure decreased with an increase in CO2 proportion; the range of decrease was larger at low CO2 proportions. CO2 proportion had little effect on the permeability rebound pressure. Besides, the larger the Langmuir volume constant of CO2, the larger was the permeability decrease coefficient and permeability rebound pressure, and the smaller the permeability recovery pressure. However, the effect of the Langmuir pressure constant on these parameters was relatively weak. Finally, in light of these results, the implications of different characteristics of permeability evolution for CO2 injection pressure adjustment in the process of enhanced coalbed methane recovery with CO2 sequestration are discussed from a macroscopic perspective. The results of this study may provide a reference to select appropriate coal seams and injection pressures for CO2 sequestration.